It has been determined by experiments that, in a large majority of color negatives, the blue, green and red spectral components of the light transmitted through the negative over the whole area thereof are substantially equal to one another, or in a constant ratio (the Evans Principle). Therefore, in most color printing systems, the printing light intensity is adjusted among the red, green and blue exposures on the basis of the red, green and blue large area transmission densities (LATD) of the original to be printed to thereby conduct color correction.
However, in the LATD printing system, the prints obtained are not always satisfactory. For instance, if the ratio of one of the three spectral components of the original to be printed is significantly greater than the ratio of the other two components, color correction based only on the LATD is unsatisfactory.
Such unsatisfactory printing is known as "color failure". When a color negative has been exposed under tungsten light illumination or fluorescent light illumination, the prints obtained are affected by the color of the light source. Moreover, when a color negative is under-exposed or over-exposed, the color balance of the prints obtained is poor because of the reciprocity law failure of the color paper, the shift of the peak of the spectral absorption of the color negative from the peak of the spectral sensitivity of the photo-receptor used in the printer, or the breakdown of the characteristic curve of the color sensitive layer at the high density region thereof.
In the conventional color printer based on the LATD system, the color prints determined to be unsatisfactory have been reprinted at another combination of the color keys selected on the basis of visual inspection by the operator. There also has been proposed a method of color correction in which color negatives are classified based on their blue, green and red large area transmission densities (LATD.sub.B, LATD.sub.G, LATD.sub.R) and the color correction is conducted in accordance with the classification.
The problem is, however, that in the above described method of classification based only on the blue, green and red large area transmission densities, the color negatives cannot be classified into proper groups. For example, in plotting the LATD.sub.B, LATD.sub.G, LATD.sub.R of negatives susceptible to green color failure in a two-dimensional coordinate system wherein one of the axes represents the density difference between the red and green densities and the other represents the density difference between the green and blue densities, LATD.sub.R, LATD.sub.G and LATD.sub.B will fall within a region 1 as shown in FIG. 1. Similarly, LATD.sub.B, LATD.sub.G and LATD.sub.R of negatives exposed under fluorescent light illumination, aged negatives, negatives exposed under tungsten light illumination and negatives exposed under low color temperature natural light in the morning or evening fall respectively within the regions 2, 3, 4 and 5 shown in FIG. 1. As can be seen from FIG. 1, the regions 1, 2 and 3 are partly overlapped with each other as are the regions 3, 4 and 5. Accordingly, it is impossible to clearly distinguish the negatives of the above groups from each other based only on LATD.sub.R, LATD.sub.G and LATD.sub.B.
In order to obtain properly corrected color prints, all the negatives should be precisely classified into groups each consisting only of negatives which can be properly corrected through the same mode of correction. For example, negatives exposed under fluorescent light illumination can be properly corrected by high correction mode, while negatives susceptible to green color failure can be properly corrected by lowered mode correction in which exposures are controlled by a black shutter. Thus, unless these two types of negatives are classified into different groups, the prints obtained will be unsatisfactory.
The object of the present invention is to provide a color correction method for color negatives which can produce satisfactory color prints from the negatives, in which the negatives are classified into groups and color correction is carried out in accordance with the group in which the color negative is classified.
Studies conducted by the inventors show that when negatives photographed by amateurs are printed by the LATD system about 35.9% of the resulting color prints are of proper color balance, about 58.3% have undesirable color balance and about 5.0% are of bad quality due to focusing errors etc. at the time of exposure. About 3-5% of the negatives were found to have such poor color balance as to produce unsatisfactory color prints when printed by the LATD system.
The negatives having undesirable color balance can be classified into a group consisting of negatives exposed under fluorescent light illumination, a group consisting of negatives exposed under tungsten light illumination, a group consisting of negatives susceptible to color failure when printed by the LATD system, a group consisting of improperly exposed negatives, a group consisting of negatives having high color temperature (such as those exposed under cloudy conditions), a group consisting of negatives having low color temperature (such as those exposed under natural light in the morning or evening) and a group consisting of aged negatives. The groups having particularly poor color balance were those consisting of under-exposed negatives, over-exposed negatives, negatives susceptible to color failure when printed by the LATD system, and negatives exposed under fluorescent light or tungsten light illumination. These groups were generally found to account for a large portion of the negatives photographed by amateurs. Therefore, if such negatives can be properly color-corrected, the yield of satisfactory color prints can be substantially raised.